1. Field of the Invention
The present invention relates to a connection structure for connecting a flexible substrate to a main substrate provided with a wiring pattern and a connection method thereof.
2. Description of the Related Art
Conventionally a flexible printed circuit is used to connect a signal line or a power line on a main substrate provided with a wiring pattern to an external circuit. The flexible printed circuit is generally a sheet-like flexible substrate and on its surface or its inner layer a wiring pattern is formed.
When a main substrate and a flexible printed circuit arc electrically connected, part of the wiring pattern formed on the main substrate and part of a corresponding wiring pattern formed on the flexible printed circuit are electrically connected by solder or the like. However, if these substrates are simply connected by solder or the like, its strength is not sufficient and there is a possibility that their joint part may come off. Therefore, methods for reinforcing the connection between the main substrate and the flexible printed circuit are proposed.
In Patent reference 1 (Japanese Utility Model Application No. S62-73576 (FIGS. 3 and 4)), as shown in FIGS. 1A and 1B (side view), resin potting which acts as adhesive is applied to reinforce the connection between a main substrate 100 and a flexible printed circuit 110. Thus, when a product including the main substrate 100 and the flexible printed circuit 110 is assembled or used, the pulling stress of the joint part is absorbed by the potting resin to mitigate stress to the solder joint part.
In Patent reference 2 (Japanese Patent Application No. H7-38253 (FIGS. 3 and 4)), the main substrate 100 and the flexible printed circuit 110 are connected as shown in FIG. 2. A copper connection foil 121 is formed on the top surface of the main substrate 100 and furthermore a resist 122 is provided on the top surface of the copper connection foil 121. A wiring pattern 123 and a cover film 124 are formed on the flexible printed circuit 110. The copper connection foil 121 and the wiring pattern 123 are connected by a thick-plated solder-pressed joint part 125. Then, adhesive 126 is spread on the contact surface between the main substrate 100 and the flexible printed circuit 110 to reinforce the joint part.
As another structure, a method for fixing the connection between the main substrate and the flexible printed circuit using rivets or the like is also proposed.
However, in the structure disclosed in Patent reference 1, it is only in the end of the resin potting area in the shorter side direction of the flexible printed circuit 110 (part enclosed by broken lines in FIG. 1A) that connection effect due to potting resin can be obtain. Therefore, sufficient reinforcement effect cannot be obtained in this structure. In the center area of the flexible printed circuit 110, the reinforcement effect fades and space efficiency becomes low.
In the structure disclosed in Patent reference 2, fairly good reinforcement effect can be obtained. However, since it is not easy to spread adhesive 126 between the main substrate 100 and the flexible printed circuit 110 after soldering, a complex process or the increase of a material cost is feared.
In the structure for fixing the connection between the main substrate and the flexible printed circuit using rivets or the like, since space is necessary for the rivets, it is difficult to miniaturize the flexible printed circuit.